1. Field of the Invention
This invention relates to circuits for controlling the energization of functional devices, and more particularly, to an energizing circuit employing a controlled switching device which effects fail-safe operation of a functional device in a system such as an automatic fuel ignition system.
2. Description of the Prior Art
Many control applications require an energizing circuit which provides fail-safe operation of a functional device, such as a relay, to provide a desired control function. The energizing circuit may include a controlled switching device, typically a silicon controlled rectifier, which when triggered into conduction effects the operation of the relay. In some control circuits, such as the one shown in the U.S. Pat. No. 3,441,356 to L. H. Walbridge, wherein a silicon controlled rectifier is used to effect energization of a relay, the silicon controlled rectifier is connected in series with the relay between outputs of an energizing source. Thus, should the silicon controlled rectifier fail in the shorted mode, the relay will be operated. In certain applications, it is desirable that the relay not be operated in the event of a failure of the controlled switching device or some other component of the energizing circuit. One example of such system is an automatic fuel ignition system in which a relay is usually employed to control the energization of a fuel valve which supplies fuel to a burner apparatus for ignition by an ignition circuit. In such application, a failure of the controlled switching device, for example, could enable the relay to be operated or enable the relay to remain operated at a time when operation of the relay is unsafe or undesirable as for example when fuel would be supplied to the burner apparatus while the ignition circuit is not operating.
To prevent such occurrence, various "fail-safe" control circuits have been proposed which are operable to maintain a switching device, such as a relay, unoperated in the event of a component failure in the control circuit. In such circuits, the operation of the relay is usually effected by a silicon controlled rectifier, and the circuits prevent unsafe operation in the event of failure of components of the trigger circuit for the silicon controlled rectifier. However, most of these "fail-safe" circuits do not afford adequate protection in the event of failure of the silicon controlled rectifier itself.
One known circuit which provides "fail-safe" protection to prevent unsafe operation due to malfunctioning circuit components, including the silicon controlled rectifier, is disclosed in the U.S. Pat. No. 3,847,533 to W. J. Riordan. The circuit, which is described with reference to an automatic fuel ignition system of the direct ignition type, causes operation of a fuel valve through the discharging of a capacitor which is fully charged during each positive half cycle of an AC signal. A first silicon controlled rectifier which is fired periodically during each positive half cycle whenever a flame is not established, effects the discharge of the capacitor to operate the valve to supply fuel to a fuel outlet for ignition. A second silicon controlled rectifier is fired periodically during each negative half cycle when a flame is established to effect the discharge of the capacitor to maintain the valve operated. Since the circuit requires two silicon controlled rectifiers to permit initial operation of the valve and to maintain the valve operated, thereby it would appear the possibility of failure of the control device is increased. Also, while pulsed operation of the silicon controlled rectifiers is used to control the discharge of the capacitor, and lack of pulsing is indicative of a component failure, the circuit does not appear to employ the capacitor to prove the operability of the silicon controlled rectifiers.